The document by Gunnar Prytz and Jonny Skaalvik “Redundant and Synchronized EtherCAT Network”; IEEE Embedded Systems (SIES) 978-1-4244-5841-7/10, 2010 describes redundant EtherCAT networks. A master having two embed slaves was suggested by Prytz. There are two effects of integrating two slaves in the master: The master may share the same HW clock with the embedded slaves, i.e. providing a reference clock. Furthermore, redundant network topologies are available, in particular a ring topology starting at the first embedded slave and terminating at the second embedded slave, so as to have a closed loop connection, via the sequence of slaves, from the first embedded slave to the second embedded slave, as shown in the left part of FIG. 2 of Prytz. When a connection in the ring is broken, two independent subsequences remain, each connected to a respective embedded slave, as shown in the right part of FIG. 2 of Prytz. Hence communication from the master to any of the slave in the sequence is still available.
However, the suggested schema in the above publication will not be able to support both redundancy and Distributed Clock (DC) synchronization without a major effort in the master side